Solvent-based, water-based, and solventless urethane resin compositions have been known as the urethane resin compositions and used in various usages such as forming materials for films, sheets, etc., adhesives, and coating materials.
Among these, solvent-based urethane resin compositions are capable of forming coatings that generally have excellent durability and water resistance compared to water-based urethane resin compositions and are thus used widely even today.
However, under the recent trends of reducing the impact on the environment, there is a tendency to avoid use of solvent-based urethane resin compositions. In particular, since urethane resin compositions containing strong solvents such as dimethylformamide have strong adverse effects on the environment and human bodies, the industry requests development of resin compositions that will replace the solvent-based urethane resin compositions.
Recently, development of urethane resin compositions that use alcohol solvents having lower impact on the environment than dimethylformamide and the like is increasingly pursued. For example, an alcohol-soluble urethane resin composition that contains an alcohol having 1 to 7 carbon atoms and a polyurethane resin having a hydrolyzable silyl group in a molecular terminus or a side chain prepared by a particular production method is known (e.g., refer to PTL 1).
Meanwhile, as the usage of the urethane resin compositions expands increasingly, the fields of automobile parts, home electric appliances, electronic materials, and the like in some cases require urethane resin compositions that can form coatings with significantly high heat resistance so that the coatings do not undergo deformation or mass loss even in a high-temperature environment of about 200° C. or higher and preferably 260° C. or higher.
However, the incipient fluidization temperature of a cured product obtained by curing a urethane resin composition described in PTL 1 is generally about 150° C. to 200° C. and thus the composition can rarely be used in usages that require the aforementioned high level of heat resistance.
As a method for improving the heat resistance, a method of introducing a large number of functional groups that can form crosslinking points in a urethane resin has been known.
However, coating materials and adhesives are generally required to exhibit excellent storage stability that does not cause gelation or the like even when left at room temperature. The urethane resin composition having a large number of crosslinking points introduced therein rarely has storage stability and coating workability sufficient for practical application.